Planetary gear sets are commonly used in transmissions and speed reduction devices in motor vehicle applications. It is known in such applications to use compound planet gears. Compound planet gears, as discussed herein, are gears having multiple gear elements i.e. pinions axially spaced from one another and integrally machined from a single piece of material or assembled from individual gears. Typically, a compound planet gear will have a large diameter element and one or more axially spaced smaller diameter element(s) disposed around a common axis. The use of such compound gears facilitates the manufacture of compact planetary gear sets.
It is desired in the manufacture of motor vehicles to reduce the noise, vibration, and harshness (NVH) characteristics of the power train. Previously, it has been known to use compound spur gears in planetary gear sets. However, spur gears have an inherent noise associated with them in operation. Further, regular spur gears have teeth that come into contact with mating gears along the entire face width and along a line parallel to the axis at all times. Helical gears on the other hand start contact at an interface point at one extreme and progress across the tooth width to an interface point at the other extreme. This results in the ability to operate helical gears at higher speeds and at greater loads than equivalent spur gears, while doing so in a smoother and quieter manner. Accordingly, it is preferred in many planetary gear applications to forego the use of conventional spur gears in favor of helical planet gears. However, the use of helical gears has certain disadvantages of its own. Because the force transmitted between teeth of meshing helical gears is always normal to the tooth surfaces, helical gears generate a component force along the axis of the gear causing end thrust. In the past the problem of end thrust has been dealt with in many ways including the use of herringbone gears and/or thrust bearings. However, such solutions are expensive and impractical in many situations. It has been discovered that axial gear forces produced by helical gears can be canceled in part by utilizing compound planet gears wherein the gear elements have different leads. The use of compound helical planet gears having gear elements with different leads also provides the advantage of allowing the individual gear elements to be designed independently of one another so as to optimize the performance capabilities of the planetary gear set. The use of such gears has previously been impractical due to the need to maintain timing of planet gears to share load equally and the lack of adequate manufacturing techniques to do so. As used herein the term “timing” refers to the relationship of one gear element to another. Simply put, timing of the multiple gear elements of a compound gear is the relationship that ensures that each gear element properly engages with its corresponding mating or meshing gear as the multiple gear elements are turning together, so that loads are properly shared without excess wear etc. While compound planet gears having gear elements with different leads can be timed during initial machining, most gear finishing processes currently used tend to change tooth profiles and axial spacing of the gear elements resulting in less than satisfactory timing. As a result of inaccurate timing gears fail. Improper manufacturing attempts have resulted in edge loading and ultimately gear failure. Accordingly, there is a clear need in the art for a method of manufacturing compound helical planet gears having different leads that maintains the timing between the compound gear elements and attains equal load share among the multiple planet gears of a planetary gear set.